While Itagaki has spoken about his feelings regarding the PSP and DS in the past, he has gone all out in his latest interview with Gamasutra. See, if he was just going to make a game on the PSP, he could've just made a PS2 game instead. No, to him, the user input is integral in creating a unique and lasting experience.
Enter the DS, where the stylus-driven combat of Ninja Gaiden: Dragon Sword is exactly what Itagaki was referring to. While the graphics might not be up to par for those who've enjoyed his games of late, Itagaki claims that the controls should more than make up for it.
It's an incredible interview, one that you should read immediately.